The present invention relates to rotating anode x-ray tubes and is particularly related to a drive apparatus for rotating the anode within an evacuated envelope. The present invention also relates to apparatus for electrically isolating the anode from at least a portion of the rotary drive producing elements located within the evacuated envelope.
Typically, a rotating anode x-ray tube includes an evacuated envelope, a cathode assembly, a rotating anode assembly, a bearing assembly to facilitate anode rotation and an induction motor to drive rotation of the anode. The induction motor includes a stator located external the evacuated envelope and a rotor attached to the anode assembly and located within the envelope. Energizing the stator coils causes the rotor of the induction motor to rotate the anode in the bearing assembly, as more fully described below.
During production of x-rays, a current is passed through a cathode filament located in the cathode, heating the filament such that a cloud of electrons is emitted, i.e. thermionic emission occurs. A high electrical potential, on the order of 100-200 kV, is applied across the cathode assembly and the anode assembly. The high voltage potential accelerates the electrons and causes them to flow in an electron beam from the cathode assembly to the anode assembly. A cathode cup focuses the flowing electrons onto a small area, or focal spot, on a target of the anode assembly. A portion of the x-rays pass through one or more x-ray transmissive windows of the envelope and an x-ray tube housing.
During x-ray generation, substantial heat is generated by the electron beam striking the anode. In order to distribute the thermal loading created during the production of x-rays, a rotating anode assembly configuration has been adopted for many applications. In this configuration, the anode assembly is rotatably supported by the bearing assembly. As the electron beam strikes the anode, the anode is rotated in the bearing assembly by the induction motor about an axis such that the electron beam strikes a continuously rotating circular path about a peripheral edge of the rotating anode. The portion of the anode along the circular path that is being struck by the electron beam becomes heated to a very high temperature during the generation of x-rays. The rotating anode is thus cooled before returning to be struck again by the electron beam.
As described above, a high electrical potential difference is applied across the anode assembly and cathode assembly during the production of x-rays. The rotor of the induction motor attached to the anode assembly is at the same high electrical potential as the anode assembly. Raising the rotor to the anode potential presents a problem since the stator of the induction motor, located outside the x-ray tube evacuated envelope, is at a different electrical potential, usually ground. Because of the large potential difference between the stator and rotor, the stator and rotor have to be spaced apart such that arcing between the two motor sections does not occur. However, the greater the spacing between the rotor and stator, the greater the reluctance between the rotor and stator. Higher reluctance reduces the efficiency of the motor. The reduced efficiency of the motor usually results in the following disadvantages: (i) designed oversizing of the motor to meet anode rotation requirements, (ii) excess heat generation in the rotor due to eddy currents, (iii) longer time to reach operational speed of the rotating anode, (iv) decreased x-ray tube and bearing life, and (v) added cost of manufacture and operation. As the need for higher power x-ray tubes increases, larger anodes used to meet those needs will further exacerbate these problems. Larger anodes, with increased moments of inertia, require more force from the induction motor to accelerate quickly to operational speeds.
Some of the disadvantages listed above are interrelated, for example, slower acceleration of the anode induces more heat in the rotor of the x-ray tube. The rotor heat, in addition to the heat transferred from the anode during normal operation, can migrate to the bearings which can result in reduced lubricant efficiency due to evaporation of the lead and silver ball bearing lubricant. Reduced lubricant efficiency is detrimental to tube and bearing life.
As the anode accelerates to operational speed, it passes rotational speeds that create major mechanical resonances in the rotating components of the tube. Less efficient motors, having slower acceleration of the anode to operational speed, increases the amount of time that the anode experiences these major mechanical resonances. This factor also increases mechanical wear of the bearings and has an undesirable effect on tube and bearing life.
The present invention is directed to an x-ray tube that satisfies the need to provide a rotating anode x-ray tube which has improved motor efficiency. The present invention also provides for alternate drive apparatus configuration for rotating the anode. An apparatus in accordance with one embodiment of the present invention includes an x-ray tube having an evacuated envelope. Inside the evacuated envelope are a cathode and a rotatably mounted anode. The apparatus includes a rotor for rotatably driving the anode. The rotor is electrically insulated from the anode by mounting the rotor to the anode assembly using an electrically insulating member.
In accordance with a more limited aspect of the present invention, the apparatus includes a stem attached to the rotatably mounted anode and the stem is rotatably supported in the evacuated envelop by at least one bearing. The bearing has an outer bearing race member. The evacuated envelope of the x-ray tube includes a cylindrical wall portion and the outer bearing race member of the bearing is received along an inner surface of the cylindrical wall portion.
In accordance with another limited aspect of the present invention, the electrically insulating member is a ceramic material, preferably Alumina.
In yet another limited aspect of the present invention, the rotor is a disk configuration.
In accordance with a more limited aspect of the invention, the rotor includes permanent magnets.
In accordance with another aspect of the present invention, the apparatus includes a drive member external to the tube, the drive member including means for providing a magnetic field coupled to the rotor.
In accordance with another aspect of the present invention the external drive member includes any of a fluid drive, a gear drive, a belt drive, a DC motor, or a pancake wound stator.
In accordance with another aspect of the invention, a method is provided for rotating an anode supported by a bearing assembly and attached to an internal rotatary drive member. The bearing assembly and internal rotating drive member are located in an evacuated x-ray tube envelope. The method comprising the steps of electrically insulating the internal rotatary drive member from the anode. A magnetic field of the internal rotatary drive member is magnetically coupled to a magnetic field of an external drive member. Finally, the step of rotating the magnetic field of the external drive member is implemented to rotate the anode.
In accordance with a more limited aspect of the method, the step of magnetically coupling includes the step of utilizing permanent magnets on at least one of the internal and external drive member.
In accordance with yet another limited aspect of the inventive method, the step of magnetically coupling includes the step of utilizing stator windings in the external drive member.
In accordance with another limited aspect of the inventive method, the step of rotating the magnetic field of the external drive member includes generating a magnetic field in a stator winding located outside the x-ray tube envelope.
According to yet another limited aspect of the method, the step of magnetically coupling includes the step of inducing the magnetic field of the internal drive member.
And, in another limited aspect of the invention, the step of rotating the magnetic field of the external drive member includes the step of pumping a fluid through a fluid drive device.
One advantage of the present invention is that by electrically isolating the rotor from anode potential, the stator and rotor can be closer without arcing. Closer spacing between the stator and rotor reduces reluctance and thereby improves motor efficiency. A more efficient motor allows quicker anode acceleration to operating speeds. Faster acceleration to operating speeds allows the anode to pass through major resonances thereby reducing mechanical wear to the bearings. This also reduces electromagnetically induced heating effects in the rotor.
Another advantage of the present invention is more effective cooling of the rotor bearing assembly. By moving the stator and rotor away from the bearings the bearings may be in direct contact with the tube envelope, thereby conductively cooled by the oil within the housing. In addition, removing the rotor from around bearing assembly removes this heating source from contact with the bearing assembly.
Another advantage of the present invention is that larger bearings may be used in the bearing assembly since the rotor drive components are not surrounding the bearing assembly within the neck of the x-ray tube. Larger bearings can more effectively handle the larger mechanical loads that accompany larger anodes and faster gantry rotational speeds. This increases the life of the x-ray tube.
Yet a further advantage of the present invention is that the rotor may be driven by alternate drive methods such as a pancake stator, permanent magnet or fluid drive mechanisms.
The present invention provides the foregoing and other features hereinafter described and particularly pointed out in the claims. The following description and accompanying drawings set forth certain illustrative embodiments of the invention. It is to be appreciated that different embodiments of the invention may take form in various components and arrangements of components. These described embodiments are indicative of but a few of the various ways in which the principles of the invention may be employed. The drawings are only for the purpose of illustrating a preferred embodiment and some alternate embodiments. The drawings are not to be construed as limiting the invention.